The present invention relates generally to optical devices and more particularly to a zero-displacement phase retarder device and method.
Phase retarders are optical elements designed to change the phase shift between the two orthogonal polarization components of a polarized optical beam, without affecting their relative amplitude. These elements are primarily used in the synthesis and analysis of the polarization state of light.
A very common application of phase retarders is the conversion of linearly polarized light into circularly polarized light. Introducing a 90xc2x0-phase shift between the two orthogonal polarization components does this. The 90xc2x0-phase retarders (also called quarter-wave plate) are applied as optical isolators used in laser interferometry, ellipsometry, electro-optic modulators and so on.
In material processing applications, such as cutting and drilling, circularly polarized light is preferred over linearly polarized light because it makes the cutting and drilling isotropic, that is independent of the cut direction. Holes that are drilled with circularly polarized light are circular, whereas those drilled with linear polarized light will be elliptical.
In general, there are several mechanisms to obtain the retardation effect. These mechanisms include the propagation of light through a linear birefringent medium, total internal reflection, reflection at the Brewster angle, reflection by a film-coated substrate and others. Reference can be made to R. M. A. Azzam and N. M. Bashara, xe2x80x9cEllipsometry and Polarized Lightxe2x80x9d, Chapter 5 xe2x80x9cInstrumentation and Techniques of Ellipsometryxe2x80x9d, Ed. North-Holland Publishing Company (1977) for details regarding prior art mechanisms. Among these known mechanisms, the first one is generally the most attractive because the retarder does not disturb the direction of light propagation. It only changes the polarization state.
The simplest retardation plate is a uniaxial crystal cut to include the crystalline optic axis direction. If the thickness of the plate is xe2x80x9cdxe2x80x9d, the difference in speed between the ordinary and extraordinary components leads to a cumulative phase shift xcex4 equal to (2xcfx80/xcex)(no-ne).d, where xe2x80x9cxcexxe2x80x9d is the free-space wavelength, and no and ne are the ordinary and extraordinary refractive indexes of the crystal, respectively. For the mid- and far-IR region (e.g., from 2-20 xcexcm), inexpensive commercially available crystals with a large difference between no and ne do not exist.
In the mid- and far-IR region, one prefers to produce phase retarders (in particular 90xc2x0-phase shifters) based on the reflection of light from a metallic surface. These retarders are typically made on a copper substrate mirror coated with a dielectric multilayer structure with layer thicknesses designed to function as a quarter-wave plate at a particular wavelength, i.e. to transform the linearly polarized light into circularly polarized.
Unfortunately, the optical arrangement used to obtain a circularly polarized beam from a linear polarized beam completely disturbs the setup that was aligned for the linearly polarized beams. To correct the setup, two angles must be correctly set. First, it is necessary to have an angle of incidence equal to 45xc2x0, and second, the plane of polarization of the incident beam must be 45xc2x0 with respect to the plane of incidence. Any changes or corrections needed to be made to the phase leads to the changes in the direction of light propagation. Hence such element is very impractical for using in an optical setup because any required change in the polarization or its orientation, drastically disturbs the optical arrangement.
Transmissive phase retarders have the advantage that they do not change the plane of propagation. Such phase retarders are the subject of U.S. Pat. Nos. 4,536,063 and 4,514,047. Because the multi-layer phase retarders do not operate under normal incidence, they induce a lateral shift of the laser beams after insertion of the element. This lateral shift is different for each wavelength. Hence two laser beams originally aligned will be shifted with respect to each other after passing though the phase retarder.
In U.S. Pat. No. 4,930,878 improvements are suggested for the well known Fresnel Rhombus. The Fresnel Rhombus is described by J. M. Bennett, Appl. Opt. 9 (1970) 9, pages 2123-2129, September 1970. It is clear that the prismatic element from the ""878 patent also induces lateral shifts for each different wavelength.
In the invention described in U.S. Pat. No. 4,917,461 the phase retarder is inserted orthogonal to the beam propagation. This feature provides an advantage as it simplifies the arrangement procedure. But this phase retarder has the disadvantage that the axis of the optical beam needs coincide with the optical axis of the polarizing element. If not, the beams will be displaced in the vertical direction. Any vibration in the room of the optical arrangement will disturb the propagation direction of the laser beams after passing through the phase retarder.
U.S. Pat. No. 4,514,047 discloses an on-axis achromatic quarterwave retarder prism. This prism has a shape similar to a dove prism with three internal reflections and is designed to have the emerging light beam on the same axis as the entering light beam only when the light beam is incident along the optical axis. In all other cases the outgoing beam is only collinear but displaced with respect to the incoming beam. This patent claims that the device operates over a wide range of wavelengths of light.
The present invention provides optical components that are advantageous over the optical components of the prior art. In one aspect, the invention is directed to an optical arrangement aligned for multiple wavelengths where the insertion of a phase retarder in the beam propagation path does not affect the alignment of the multiple laser beams. More in particular this embodiment of the invention relates to applications where a polarization change of an invisible laser beam is required when this beam is already aligned with a visible laser beam.
In one aspect, the present invention discloses a zero displacement phase retarder that includes three prisms and two reflecting surfaces. These elements induce a phase shift of one component of polarized light with respect to the orthogonal component. In the preferred embodiment, the three prisms are mounted with their flat bases on a substrate surface that includes the two phase-shifting reflectors. The two outer prisms have a flat surface perpendicular to the beam propagation path. The third surface makes a predefined angle with respect to the surface normal to the incoming (or outgoing) beam. The central prism can be considered as the combination of the two outer prisms unified at their flat surface.
In the preferred embodiment, the prisms are at an equal predefined distance from each other. The aperture of the phase retarder is only slightly smaller than the prisms. Any multi-color beam incident within the aperture of the optical element will leave the element without any lateral displacement.
One application where the present invention has particular use is in laser cutting and laser drilling. In these applications, when linearly polarized light is converted into circularly polarized light, the cutting efficiency decreases along the direction that is coincident with the originally linearly polarized state but increases along the orthogonal direction. Hence when different applications are to be executed with the same laser processing tools, it is interesting to switch effectively between linearly and circularly polarized light. The preferred embodiment of the present invention provides the benefit that one can insert the phase retarder into the optical setup without affecting the propagation paths of the visible and invisible laser beams.
The preferred embodiments of the present invention have other advantages over prior art devices and methods. In general, prior art phase retarders effect the propagation path once they are inserted in the optical arrangement and hence every new alignment is very time consuming. One of the advantages of the present invention is that the phase retarder can be used for visible and invisible light. After the phase retarder is included in the optical arrangement, the xe2x80x9cvisiblexe2x80x9d and xe2x80x9cinvisiblexe2x80x9d light beams first remain coincident and second do not show any lateral displacement. It is also advantageous that the device does not have an optical axis and therefore as long as the beam enters the device orthogonally, vertical or horizontal displacements will not affect the operation.
By combining two phase changing reflectors in a single optical device, whereby each has a different central operation wavelength, the degree of achromaticity of such optical device is extended with respect to an optical device comprising one single phase changing reflector.